A system model of a signal communications system is as shown in FIG. 1. At a transmitting end, signal modulation is performed on primary data subjected to channel coding, so as to modulate the primary data into an audio signal of a data frame composed of a plurality of symbols and conduct digital-to-analogue conversion on the signal, and finally the signal is transmitted through a loud speaker. At a receiving end, a microphone after receiving the audio signal, reduces the audio signal to the primary data through analogue-to-digital conversion, signal demodulation and channel decoding.
Audio communications with a frequency below 24 kHz may make full use of present video and audio playing devices and receiving devices without specially customized loud speaker and microphone. However, in the natural world, an audio with a frequency below 24 kHz usually has very strong interference caused by environment noises.